Many discharge-based extreme ultraviolet (EUV) sources require the launching of high currents (10 kA or more) off electrode surfaces [for example, U.S. Pat. No. 5,504,795 “Plasma X-Ray Source”, McGeoch (1996); U.S. Pat. No. 6,541,786 “Plasma Pinch High Energy with Debris Collector”, Partlo et al., (2003)]. A principal and long-standing problem associated with this activity is a degree of electrode heating and erosion that limits the peak current, pulse duration and pulsed operating life of such devices. The default mode at very high current is “super-emission” of electrons from an extremely hot surface created by ion bombardment but this condition is still accompanied by evaporation of electrode material. In a previous US patent filing [U.S. application Ser. No. 12/854,375 “Z-Pinch Plasma Generator and Plasma Target”, McGeoch (2010)] there has been disclosed a magnetically-assisted cathode with two advantages over conventional cathodes. Firstly, azimuthal drift of electrons in the crossed electric field of the plasma-electrode sheath and the applied magnetic field spreads the current very uniformly, thereby eliminating surface hot spots. Secondly, the spiralling path of surface secondary electrons produces more efficient ionization by maintaining the electron energy close to the energy of the maximum ionization cross section, so ion impacts on the surface that produce secondaries are less energetic, and hence there is reduced sputtering and surface heating. In the applicant's laboratory, cathodes based on this principle have produced >8 kA current pulses of 2 μsec duration for more than 100 million pulses with negligible surface erosion, in a Z-Pinch plasma-generating device running on a mixture of helium and lithium.
The magnetically-assisted cathode has been operated in a concave cathode configuration for Z-pinch generation [U.S. application Ser. No. 12/854,375 “Z-Pinch Plasma Generator and Plasma Target”, McGeoch (2010)]. While this approach confers the above advantages of uniformity and more efficient electron amplification, it does not provide magnetic shielding of the electrode from the converging hot plasma. Also, the concave approach does not provide focusing of the compressed gas in any more than two dimensions, i.e. a cylindrical plasma shell is compressed without length change onto the axis of the device, so the line density of the pinch is limited. However, when extreme ultraviolet light (EUV) generation from lithium vapor is attempted, a high line density is needed in the pinch, and it is difficult to arrange this via two-dimensional compression alone, because of a limited available lithium vapor pressure.